The invention concerns a method and apparatus for heating glass melting furnaces.
The main problems in the construction and operation of glass melting furnaces concern not only a reduction in the specific energy consumption per ton of glass (energy optimization), but also a reduction in the environmental emissions and retention of the furnace condition. The most polluting components in the waste gases are NOx and CO, both highly poisonous compounds, and also soot and hydro-carbons.
In order to provide a better understanding of the problems which arise in glass melting furnaces where air is preheated in regenerators, the following relationships are explained. Pairs of burner ports are operated in a rhythmic reversing cycle. This applies to U-flame furnaces, in which the burner ports are installed adjacent to one another at one end of the furnace, and also to cross-fired furnaces, where the burner ports are installed on opposite sides of the furnace. During the firing phase, fuel is mixed with the preheated combustion air from the corresponding regenerator and heats the furnace. Simultaneously, heat is removed from the material forming the regenerator. During the reversed phase, the very hot furnace waste gases are led back to the same regenerator through the same burner port, and the regenerator material is heated up again, and so on.
However, if sub-stoichiometric (fuel rich) conditions exist in or near the flame, soot is formed that is deposited in the area of the burner ports or port necks. On the one hand, with sub-stoichiometric conditions in or near the flames, followed by stoichiometric completion of the combustion, a desirably low NOx content is achieved in the waste gases. On the other hand, undesirable soot is formed locally. These problems and the measures for avoiding them are, to a certain extent, diametrically opposed and the invention presented here attempts to find an advantageous method of solving the problems.
In the HVG-Mitteilung (=HVG-Newsletter) No. 1894 (pages 1894-1 to 1894-20) Jütte writes about a paper given at the Technical Committee VI (=Fachausschuss) of the German Glass Technical Society (=DGG) in Würzburg on 19th Mar. 1997 under the title “Experiences with primary side NOx reduction measures on a regenerative cross-fired furnace”, stating that the greatest influence on the formation of NO is the temperature, which often masks the influence of the oxygen concentration.
When writing in the HVG-Mitteilung (=HVG-Newsletter) No. 1127 (pages 261 to 275) about a paper given on 20th Oct. 1969 Hein and Leuckel had previously stated that only flames with early and stable ignition exhibit strong carbon radiation. Special burners and their operating parameters are quoted as remedial measures.
GASWÄRME International (=Gas Heat International)-49 (2000) issue 4/5-April/May (pages 207 to 212) contains an article by Ahmad Al-Halbouni with the title: “Continuous air graduation: A new way of controlling the combustion and emissions behavior of gas flames”. On page 208 a diagram is used to explain an “NO—CO relationship”, whereby the diagram shows clearly that at low temperatures in the combustion chamber CO, soot and CxHx are the most common products, while at high temperatures thermal NO and NO from N2O predominate. It would therefore appear that the possibilities for reducing these components by varying the temperature in the combustion chamber contradict one another. Special burners and their operating parameters as remedial measures are also quoted here.
Further melting parameters of glass melting furnaces must also be taken into consideration, such as the melting out of the charging material (batch, glass cullet), which floats on the melt, the subsequent refining and conditioning of the melt and the adjustment of the optimum melt temperature for the further processing of the glass.
From patent DE 42 18 702 C2 and the corresponding EP 0 577 881 B1 and U.S. Pat. No. 5,755,846 it is known that, in order to reduce the nitrogen oxide emissions in glass melting furnaces heated with fossil fuels, flames with either over-stoichiometric (oxygen rich) or sub-stoichiometric (fuel rich) mixtures of fuel and oxidation gases with a cascaded flame arrangement can be created, whereby when the combustion gases have been mixed, the complete combustion process is more or less stoichiometric. The primary fuel, the largest component of the fuel requirement, is supplied through underport burner nozzles and the secondary fuel is supplied through burner nozzles, the so-called cascade burners, installed in the sides of the so-called burner ports, through which all the pre-heated combustion air from the regenerators flows into the combustion chamber above the underport burners. Although this method still operates successfully, the glass industry is now looking for a further improvement in combustion behavior. The combustion air has the tendency to divert the flame from the cascade burner shortly after entry into the combustion chamber, thereby making it more difficult to mix the gases in the flames.
From patent DE 42 44 068 C1 it is known that, in order to reduce nitrogen oxides in the waste gases of a glass melting furnace, a stepped area with two side walls can be installed at the furnace end of the port neck, and a gaseous fuel blown into the stepped area from at least one side through a fuel gas nozzle. The edge of the step causes a disturbance in the flow and a vortex in the combustion air supplied through the port neck, and pre-combustion in the form of a rolling flame is created with insufficient air, i.e., sub-stoichiometric combustion, the flue gases and flames of which lie as a dividing layer between the combustion air supplied through the port neck and the at least one flame of the relevant underport firing, whereby the combustion of the fuel from the underport burners is delayed. The resulting reduction in the highest temperatures leads to the aforementioned reduction in the formation of nitrogen oxides. However, it has been shown that, as a result of the lack of air in the rolling flame, soot is formed which is deposited as an increasingly thick graphite layer in the stepped area. When the firing is reversed, graphite flakes break off from time to time and are drawn in the opposite direction to the previous air flow through the port neck into the regenerator, where they collect. The regenerator is normally connected to a high-voltage electrostatic precipitator, and graphite particles which finally reach the electrostatic precipitator cause short circuits.
From the paper by Becher and Wagner “The second generation of the SORG® Cascade Heating System for reducing NOx in combination with additional primary measures” presented to the Technical Committee VI (=Fachausschuss) of the German Glass Technical Society (=DGG) on 10th Oct. 2000 in Würzburg, it is further known that cascade burners can be installed in the side wall of a step, which extends across the end of the burner port just before said burner port opens into the furnace. This produces an area sheltered from the combustion air, allowing the cascade flame to increase in length across the burner port and improves the mixing of the flame gases.
However, operation of such systems has shown that soot deposits form in the burner ports and in the steps on the firing side, where the soot deposits sinter to form graphite layers. Alternating firing reversal takes place in such furnaces, which can be either U-flame or cross-fired installations, and these graphite layers tend to break off as a result of thermal deformation when the firing is reversed, so that graphite splinters collect in the regenerators. These splinters are swept by the high velocity waste gases into the electric particle separator connected to the regenerator, where they can cause short circuits. Therefore it was considered for some time whether a higher nitrogen oxide content in the waste gases should be permitted in order to reduce the graphite formation, as it can be presumed that these demands are basically diametrically opposed.
From U.S. Pat. No. 6,047,565 concerning the reduction of the NOx content in the waste gases of glass melting furnaces, it is known, amongst other things, that it is possible to install underport firing for the primary fuel in the lower area of a port neck below a step and to install secondary burners for the secondary fuel, amounting to 5 to 30% of the primary fuel, in a side wall or in the vertical step wall. In order to separate the lower flame of the primary fuel from the upper flame of the secondary fuel along part of the flame path, the installation of nozzles in the vertical step wall is recommended, through which an inert buffer gas, which, for example, could be carbon dioxide, is blown. As an alternative to the buffer gases, which are inert to combustion, waste gases and smoke from the furnace are suggested. On no account should these buffer gases be part of the combustion, so that the flame length and the burn-out are diverted to the center of the furnace and the flames become broader. Oxygen or gases such as air which contain oxygen are excluded as buffer gases.
Where oxygen lances are recommended, they must be installed immediately above and parallel to the glass surface and below the underport firing, so that the flame from the underport firing is diverted more to the center of the furnace in order to prevent a reducing atmosphere above the glass bath surface and discoloration of the glass. It is also stated that some of the fuel burners of the underport firing can be replaced by oxygen lances. However the total oxygen quantities emanating from the port necks and the oxygen lances must be adjusted so that the amount of oxygen supplied is less than that necessary for stoichiometric combustion.
No mention is made of either the problem of preventing or reducing soot deposits in the port necks, or the possibility of using combustion to destroy carbon deposits in the port necks and in the regenerators which may occur when the waste gas direction is reversed, which would prevent the electrostatic precipitators from being damaged by carbon splinters. In particular neither oxygen nor a gas containing oxygen, which could produce this effect, is supplied to the step described. This known solution is neither intended for nor is it suitable for this.